Linux tool? Hate when everything Unix automatically gets categorized as Linux. Have used Expect for many years for many use cases from automating servers to networking use cases as well.
For example, expect(1) is described in the 1989 Don Libes book "Life with UNIX". Libes is the author of Expect. A title such as "Life with Linux" would probably have made little sense at the time.
Correction: I went back and looked at the book and it does not mention expect. According to Wikipedia expect was first released the following year, in 1990. Apologies for the inadvertence.
Nevethless I think it is safe to say that expect was not written using or for Linux as Linux was not released until 1991.